water. The experiments in the wider flume indicated a 

 sediment- trapping efficiency for the sampler equal to 1.0. 

 Velocity profiles taken upstream from the sampler and 

 directly above the sampler were nearly identical, indicat- 

 ing that the sampler did not adversely affect the flow 

 regime. The vortex tube system operating since 1969 at Oak 

 Creek, Oregon develops vortex flow to move fluid and 

 sediment through a flume. The bed load is removed to an 

 off-channel pit where it is measured and returned to the 

 creek. The vortex flume is continually operated; sampling 

 can be continual or intermittent. The authors state that 

 the trapping efficiency for coarse sand and large particles 

 is 1.0 for all flow regimes. For lower flows, trapping 

 efficiencies are also 1.0 for finer sediments; at higher 

 flow regimes, finer sediments are transported as suspended 

 material. Sampling intervals range from a few minutes to 

 several hours, depending on the magnitude and rate-of- 

 change of discharge. The East Fork, Wyoming, conveyor-belt 

 bed-load sampler began operation in 1973 (Figure All). The 

 bed-load sampler basically consists of ' a concrete trough 

 permanently embedded into the river bed . Eight gates in 

 the trough allow all or part of the sampler to be closed. 

 Beneath the gates is a continuous rubber belt adjacent to a 

 series of belts that eventually carries sediment entering 

 the trough to a weighing scale. Sediment, after being 

 weighed, is returned to the river downstream. 



b. Indirect-measuring samplers . Similar to suspended sediment in 

 direct-measuring devices, indirect-measuring samplers measure 

 some associated characteristic of sediment movement. Three 

 types of indirect-measuring samplers have been developed for 

 riverine bed-load transport, and they are discussed below. 



(1) Acoustic sampler 



Acoustic samplers measure the sound created by particle 

 collisions with the bed material and consist of an under- 

 water microphone, located some distance above the bed, and 

 an amplifier and recorder. One type of acoustic sampler is 

 the Beauvert Laboratory hydrophonic detector, consisting of 

 a microphone mounted on a triangular base plate which rests 

 on the bed (Graf 1984) . The sound of interparticle 

 collisions and of the particle impact on the base plate are 

 amplified and recorded. Disadvantages associated with this 

 type of sampler include: (a) the apparatus can only be 

 used with larger sized material because of the weak sound 

 level produced by finer sediments; (b) large instruments, 

 while resting on the bed, may significantly alter the flow; 

 and (c) the results provide only qualitative bed -load 

 transport information (Graf 1984). 



A19 



